The present invention relates generally to the field of heat exchanger tube supports, and in particular to a new and useful tube support bar for restraining and positioning the U-bends of water tubes within a nuclear steam generator.
In a pressurized water nuclear power station, steam generators, which are large heat exchangers, transfer heat, produced via nuclear reactions in the reactor core, from a primary water coolant to a secondary water coolant that drives the steam turbine. The primary coolant is pressurized, which allows the primary water coolant to be heated in the reactor core with little or no boiling. For example, in a light water reactor, the primary coolant is pressurized to about 2250 psia and heated to about 600 deg F. in the reactor core. From the reactor, the primary water coolant flows to a steam generator, where it transfers heat to the secondary coolant. In a U-tube, or recirculating steam generator, the primary coolant enters at the bottom of the steam generator, flows through tubes having an inverted U-shape transferring heat to the secondary coolant, and then exits at the bottom of the steam generator. The secondary coolant is pressurized only to a pressure below that of the primary side, and boils as it flows along the outside of the tubes, thereby producing the steam needed to drive the turbine. Nuclear steam generators must be capable of handling large quantities of two-phase secondary coolant moving at high flow rates, and are therefore very large structures. For example, a nuclear U-tube steam generator can weigh more than 450 tons, with a diameter exceeding 12 feet and an overall length of greater than 70 feet. It may contain as many as 9,000 or more of the long, small diameter, thin-walled U-shaped tubes. For a general description of the characteristics of nuclear steam generators, the reader is referred to Chapters 47 and 52 of Steam/Its Generation and Use, 40th Edition, The Babcock and Wilcox Company, Barberton, Ohio, U.S.A., (copyright)1992, the text of which is hereby incorporated by reference as though fully set forth herein.
Heat exchangers such as nuclear steam generators require tube restraints or supports, to position the tubes and to restrain the tubes against flow induced vibration forces. In the U-bend region of a nuclear steam generator, a large flow of steam and water mixture passes upwards through the tube array, in a general direction which locally is normal to the axis of the individual U-bend tubes. This large two phase flow is able to cause excitation of the U-bend tubes via the turbulent and other flow forces imparted by the flow. As a result, the tubes tend to vibrate in both the out-of-plane and in-plane directions relative to the U-bend plane. Typically this restraint function is provided by an array of flat U-bend support bars. While such flat bars provide positive restraint in the U-bend out-of-plane direction, they provide restraint only by friction in the in-plane direction.
One known type of nuclear steam generator U-bend support, depicted in FIG. 1, and in greater detail in FIG. 2, is manufactured by Babcock and Wilcox Canada Ltd. FIG. 1 shows a nuclear steam generator 100 having a plurality of U-bend tubes 110. The U-bend tubes 110 are arranged in layers, with each layer having multiple tubes all positioned within the plane of their respective U-bends. Each layer incorporates a set of tubes of successively larger radius which are nested to create the layer of tubes in the particular plane. For purposes of illustration, however, FIG. 1 shows only a limited number of U-bend tubes 110, and FIG. 2 shows only the outermost tubes of the center U-bend layers. The straight leg portions of the U-bend tubes 110 are supported at several locations by vertically spaced apart tube support plates 200, as shown in FIG. 1.
As shown in FIG. 1, and in greater detail in FIG. 2, the U-bend tubes 110 are positioned and restrained in the U-bend region by a U-bend support assembly 120 which includes a number of U-bend support bar arrays 180. Each U-bend support bar array 180 is comprised of flat U-bend support bars 160, which are positioned in sets between layers of tubes within the U-bend region of the steam generator. The flat U-bend support bars 160 fan out from the center of the U-bend such that individual bar sets are assembled into a U-bend support bar array 180, or xe2x80x9cfanxe2x80x9d bar array, in which the lower ends of the individual bars are interconnected. As shown in FIG. 2, the inner ends of the flat U-bend support bars 160 of a particular U-bend support bar array 180, are interconnected by a mechanical or welded joint 210. Each U-bend support bar array 180 incorporates about 4 to 12 of the flat U-bend support bars 160. The flat U-bend support bars 160 are positioned so as to provide support to the U-bend tubes 110 at certain points along the arc of each U-bend tube in the array. The angular separation of the flat U-bend support bars 160 depends upon the U-bend size and flow conditions, and the flat U-bend support bars 160 are located to minimize unsupported tube lengths. The individual flat U-bend support bars 160 are typically made of stainless steel, and are about 1xe2x80x3 to 1.5xe2x80x3 wide and about 0.1xe2x80x3 to 0.2xe2x80x3 thick. A U-bend support assembly 120 may incorporate 100 to 200 of the fan-shaped U-bend support bar arrays 180, with one such array located between each plane of U-bend tubes. The outer ends of the flat U-bend support bars 160 are collected, restrained and supported by arch bar support structures 170 located adjacent the steam generator U-bend. Each arch bar support structure 170 positions the flat U-bend support bars 160 of a U-bend support bar array 180, carrying the weight of the bars and redistributing the weight of the U-bend support assembly 120 back to the peripheral layer of U-bend tubes.
The U-bend support bar arrays 180 position the planes of U-bend tubes 110 in space, and most importantly, restrain the individual U-bend tubes against flow induced vibration. Restraint against out-of-plane motion is provided by the physical presence of the flat U-bend support bars 160, which are situated immediately adjacent to the U-bend tubes 110. The bar-to-tube clearance is purposely quite small, with the bar-to-tube diametral clearance varying from about 0 to 0.010xe2x80x3 or more. The flat U-bend support bars 160, with their small bar-to-tube clearances, thus prevent significant motion of the tubes in the out-of-plane direction 140. In the in-plane direction 130, however, the U-bend tubes 110 are not positively restrained, but instead depend solely upon friction between the U-bend tubes 110 and the flat U-bend support bars 160 to restrict and dampen the flow induced motion of the tubes in their in-plane direction. Depending on the design details and flow conditions, the effect of the friction in providing in-plane restraint may not be fully adequate in providing in-plane restraint.
The present invention is drawn to an improved heat exchanger tube support bar which is particularly suited for the U-bend region of a U-tube nuclear steam generator. The bar is configured so that it has scalloped pockets on opposite surfaces of the bar, which can be positioned to engage and restrain the tubes during steam generator operation. Each pocket is arranged so that it provides a support surface for a tube. As the tube is supported on each side by one of these pockets with close clearance, the tube is positively constrained in its out-of-plane direction by virtue of the small tube-to-bar clearance, and positively supported in the in-plane direction by the contour of the pocket as it engages with the U-tube.
The support bar has a thin profile width section whereby the bar may be positioned within an existing tube array in two different orientations, i.e. with either the diagonal or vertical cross sectional axes of the bar positioned parallel to the plane of the tubes. In the diagonal orientation, the bar may be moved, as for installation, within the space between layers of tubes without restriction. With the pocketed bars correctly positioned, and with the pocketed bars rotated to the vertical orientation, pockets along the diagonally opposite shoulder areas of the bars are able to positively position the tubes in both in-plane and out-of-plane directions for purposes of providing restraint of the tubes against flow induced vibration.
Accordingly, one aspect/object of present invention is to provide an improved heat exchanger tube support bar providing support in both the in-plane and out-of-plane directions.
It is a further aspect/object of the present invention to provide an improved U-bend support bar which reduces the susceptibility of the tubes in the U-bend region to flow induced vibration and tube fretting at the support locations.
It is an object of one embodiment of the invention to provide a U-bend support bar to serve in place of one or more of the U-bend support bars within a particular U-bend support bar array, either as a retrofit or during new equipment manufacture.
It is an object of an alternate embodiment of the invention to provide an auxiliary U-bend support bar for installation within an existing U-bend support bar array, either as a retrofit or during new equipment manufacture.
Accordingly one aspect of the invention comprises a support bar, for use in a heat exchanger having rows of tubes arranged in a plurality of parallel tube planes defining an in-plane direction and an out-of-plane direction, the support bar having an elongated body with first and second sides, and first and second ends; a first shoulder, extending along the first side adjacent the first end, and having a first plurality of pockets adapted for receiving a first row of tubes; a second shoulder, extending along the second side adjacent the second end, and having a second plurality of pockets adapted for receiving a second row of tubes; and wherein the pockets are designed to have a small clearance with the tubes after installation thereby restraining the tubes against motion in both the in-plane direction and the out-of-plane direction.
Another aspect of the invention comprises a support bar, for use in the U-bend region of a heat exchanger having rows of U-bend tubes arranged in a plurality of parallel U-bend tube planes defining an in-plane direction, and an out-of-plane direction, the support bar having an elongated body with first and second sides, and first and second ends, the body having a cross sectional form generally in the shape of a parallelogram having a short diagonal and a long diagonal, a center located at the intersection of the short diagonal and the long diagonal, a first axis passing through the center, a first pair of parallel surfaces located on opposite sides of the first axis, a second axis passing through the center, and a second pair of parallel surfaces located on opposite sides of the second axis; a first shoulder, extending along the first side adjacent the first end, and having a first plurality of pockets adapted to restrain a first row of U-bend tubes against motion in both the in-plane direction and the out-of-plane direction in the U-bend region; and a second shoulder, extending along the second side adjacent the second end, and having a second plurality of pockets longitudinally offset from the first plurality of pockets and adapted to restrain a second row of U-bend tubes against motion in both the in-plane direction and the out-of-plane direction in the U-bend region; and wherein the first and second shoulders each have shoulder sides parallel to the second axis, and inner and outer shoulder end surfaces perpendicular to the first axis.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. For a better understanding of the present invention, and the operating advantages attained by its use, reference is made to the accompanying drawings and descriptive matter, forming a part of this disclosure, in which a preferred embodiment of the invention is illustrated.